1. Field of the Invention
The present invention relates to transmitting and receiving technologies, and it particularly relates to a transmitting and receiving method, in which signals are transmitted from a plurality of antennas and the signals are received by a plurality of antennas, and a radio apparatus utilizing said method.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier communication schemes that can realize the high-speed data transmission and are robust in the multipath environment. This OFDM modulation scheme has been used in the wireless standards such as IEEE802.11a/g and HIPERLAN/2. The burst signals in such a wireless LAN are generally transmitted via a time-varying channel environment and are also subject to the effect of frequency selective fading. Hence, a receiving apparatus generally carries out the channel estimation dynamically.
In order for the receiving apparatus to carry out the channel estimation, two kinds of known signals are provided within a burst signal. One is the known signal, provided for all carries in the beginning of the burst signal, which is the so-called preamble or training signal. The other one is the known signal, provided for part of carriers in the data area of the burst signal, which is the so-called pilot signal (See Reference (1) in the following Related Art List, for instance).
Related Art List
    (1) Sinem Coleri, Mustafa Ergen, Anuj Puri and Ahmad Bahai, “Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems”, IEEE Transactions on broadcasting, vol. 48, No. 3, pp. 223-229, September 2002.
In wireless communications, adaptive array antenna technology is one of the technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the directional patterns of antennas are controlled by controlling the amplitude and phase of signals, to be processed, in a plurality of antennas, respectively. In adaptive array antenna technology, the amplitude and phase of signals transmitted from and received by a plurality of antennas, respectively, are so controlled as to form a directional pattern of the antenna. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is the MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and a channel corresponding to each of the plurality of antennas is set. That is, channels up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates. Moreover, combining this MIMO system with a technique such as the OFDM modulation scheme results in a higher data transmission rate.
In the MIMO system, the data rate can also be adjusted by increasing the number of antennas to be used for data communications. Furthermore, the data rate can be adjusted in greater detail by applying the adaptive modulation to the MIMO system. To perform such an adjustment of data rates more reliably it is desired that the transmitting apparatus already acquire from the receiving apparatus the information on data rates suited for the radio channel between the transmitting apparatus and the receiving apparatus (hereinafter referred to as “rate information”). If, on the other hand, the rate information is not transmitted periodically in the MIMO system, the transmitting apparatus transmits to the receiving apparatus a signal by which to request the transmission of the rate information (hereinafter referred to as “rate request signal”). Examples of the combinations of directivity patterns in the transmitting apparatus and receiving apparatus in a MIMO system are as follows. One example is a case where the antennas of a transmitting apparatus have omni patterns and the antennas of a receiving apparatus have patterns in adaptive array signal processing. Another example is a case where both the antennas of the transmitting apparatus and those of the receiving apparatus have patterns in adaptive array signal processing. The system can be simplified in the former case. In the latter case, however, the directivity patterns of antennas can be controlled in greater detail, so that the characteristics thereof can be improved. Since in the latter case the transmitting apparatus performs adaptive array signal processing for transmission, it is necessary to receive beforehand from the receiving apparatus the known signals by which to estimate channels. To improve the accuracy of controlling the adaptive array antennas, it is desirable that the transmitting apparatus acquire the respective channel characteristics between a plurality of antennas contained in the transmitting apparatus and those in the receiving apparatus. For this reason, the receiving apparatus transmits from all of antennas the known signals for channel estimation. In this patent specification, the known signals, for use with channel estimation, transmitted from a plurality of antennas will be called “training signals” independently of the number of antennas to be used for data communication.
Under these circumstances, the inventors of the present invention came to recognize the following problems to be solved. If any error is contained in the rate information determined by the receiving apparatus, an error will be caused in communications by a MIMO system and therefore the transmission quality and effective data rate will deteriorate. Thus, the determination of rate information by the receiving apparatus needs to be done with accuracy. In order to raise the effective data rate, it is desired that the transmission of signals other than the data, for example, the rate request signal or training signals, be minimized. When the transmitting apparatus or the receiving apparatus is powered by a battery, the lower power consumption is desired.